What does St. Bernard of Clairvaux mean by loving oneself for the sake of God, and why does he put this stage of love as the highest form of love?

St. Bernard believes that loving oneself for the sake of God places an individual at the highest form of love because they have merged themselves with transcendental truth.

St. Bernard breaks down the different forms of love that an individual experiences.  From carnal forms of affection to a conditional love of God, Bernard suggests that there is a journey through which a devotee can experience different aspects of the Lord's grace.  However, in the fourth stage of love, Bernard believes the highest form of love is evident.  In this stage, Bernard feels that there is a pure identification with God because the individual has merged both worlds of the celestial and divine:

‘My flesh and my heart faileth; but God is the strength of my heart and my portion for ever’ (Ps. 73.26). I would count him blessed and holy to whom such rapture has been vouchsafed in this mortal life, for even an instant to lose thyself, as if thou wert emptied and lost and swallowed up in God, is no human love; it is celestial.

The emphasis on "swallowed up in God" is an important element.  Bernard believes that when people love God in a manner where their own existence is absorbed in the divine, a merging takes place.  Loving God in this manner ensures that the individual is no longer separate from God, but rather within him. Unity has overcome division.

St. Bernard believes that when a person loves themselves for the sake of God, they have created a form of "heaven on earth."  St. Bernard suggests that when we focus our love on the divine, it is the highest form of expression because we have transcended temporal reality:

In Him should all our affections center, so that in all things we should seek only to do His will, not to please ourselves. And real happiness will come, not in gratifying our desires or in gaining transient pleasures, but in accomplishing God’s will for us: even as we pray every day: ‘Thy will be done in earth as it is in heaven’ (Matt. 6.10)

In this love of God, carnal affections have been surpassed.  Individuals no longer care for conditions applied to God in exchange for worship ("I pray to win the lottery" or "God, please help me get that promotion to regional manager"). Rather, this love of God is universal.  It seeks to bring the individual closer to the divine because it merges one with the force of God.  Just as God loves us selflessly, St. Bernard believes that loving God is the highest form of love because it transcends the individual.